1. Field of the Invention
The invention relates generally to semiconductor manufacturing and more specifically to a method and apparatus for providing a stable environment for a signal transmitted to assist in determining the thickness of a layer of a semiconductor substrate.
2. Description of the Related Art
During semiconductor manufacturing, the integrated circuits defined on semiconductor wafers are manufactured by forming various layers over one another. As a result of the various layers disposed over one another a surface topography of the wafer becomes irregular. These irregularities become problems for subsequent processing steps, especially processing steps for printing a photolithographic pattern having small geometries. The cumulative effects of the irregular surfaces can lead to device failure and poor yields if the surface topography is not smoothed.
A common process for smoothing the irregularities is through chemical mechanical planarization (CMP). In general, CMP processes involve holding and rotating the wafer against a polishing pad with an abrasive liquid media (slurry) under a controlled pressure. A particular problem encountered during CMP operations is the determination that an endpoint has been reached i.e., a desired flatness or relative thickness of material remaining on or removed from the semiconductor wafer has been obtained. Prior art methods include removing the semiconductor wafer to manually inspect if the wafer as well as in-situ methods using laser interferometry to measure a wafer's dimensions.
In-situ methods such as laser interferometry require the ability to “see” the wafer through the polishing pad. FIG. 1 illustrates a prior art diagram of an in-situ apparatus for measuring a thickness of a layer of a wafer 102. Wafer 102 is supported in carrier 104. During CMP operations wafer 102 is pressed against pad 106 in the presence of a slurry to planarize the wafer 102. Pad 106 sits on top of platen 108. The carrier 104 rotates the wafer 102 around its axis as illustrated by arrow 116 and the platen rotates around its axis as illustrated by arrow 114. Laser 112 is positioned to view the wafer surface through window 110 as the platen 108 rotates. European Patent Nos. EP 0,738,561 A1 and EP 0,824,995 A1 discuss in detail a laser interferometer and are hereby incorporated by reference.
A problem encountered with in-situ monitoring of CMP operations is that the environment in the gap 118 between the wafer 102 and the window 110 is constantly changing due to the dynamic environment and the abrasive nature of the process. Slurry and residue from the wafer 102 and the pad 106 are all entrained in gap 118, as well as air bubbles from the turbulence. For example, at the initiation of the CMP process the gap 118 is filled with slurry having certain optical characteristics. However, as the wafer 102 is planarized the a percentage of residue from the wafer and pad in the slurry in gap 118 becomes greater over time. Hence, the optical characteristics of the slurry in gap 118 changes, which in turn has an impact on the thickness measurement since the endpoint detector was calibrated with a slurry or fluid in gap 118 with the initial optical characteristics. While the window 110 may be located at different heights within the pad, a gap 118 will always exist so that the window 110 does not come into contact with the wafer 102. U.S. Pat. No. 6,146,242 describes an optical endpoint window disposed under a window in the polishing pad and is hereby incorporated by reference.
The non-uniform environment in gap 118 also causes noise and interference for the wafer layer thickness measurement by a laser or other in-situ method. As a result of the varying background noise and the changed conditions from the calibration, the accuracy of the thickness measurement is restricted. Furthermore, between the switching of wafers there is downtime where the slurry or residue may dry up on the window. Consequently, a film may develop over the window from the slurry sitting stagnant for a period of time. Here again, the film creates a condition which invalidates the calibration of the laser and negatively impacts the accuracy of the thickness measurement. Ultimately, the inaccuracies resulting from the background noise or the changed calibration parameters translate into a thickness measurement which is not representative of the wafer being planarized which in turn leads to poor yields and even device failure.
In view of the foregoing, there is a need for an apparatus and device which provides a stable background environment for measuring the thickness of a layer of a semiconductor wafer during CMP operations.